With immortality comes immobility.
Because some quaking aspen trees can reproduce by copying themselves, some people have wondered whether they might live forever, at least theoretically. But even if that's possible, they're still not immune from the ravages of time.
As aspen clones grow older, a slow buildup of genetic mutations impairs their pollen production. After a few tens of thousands of years, they won't produce any pollen at all.
When that happens, trees will still be able to sprout clones from their root tips, but they won't be able to make seeds. They'll be stuck in place, vulnerable to disease or disaster.
“There’s a slow and steady loss of fertility with age,” said San Diego State geneticist Dilara Ally. “Because we were able to calculate the rate at which male fertility was lost, we could estimate how long it would take fertility in the oldest clone to dwindle away entirely.”
From the time they're seedlings, quaking aspens like those studied by Ally reproduce clonally, sprouting new trees from specialized root tips. As they mature, they also start to self-fertilize, producing clonal seeds. In both cases, the resulting tree is a copy of the original.
Individual trees have a lifespan of about 200 years, but clones -- scientists consider the collective as as single entity -- can sprawl for acres, all descended from one original tree, and apparently able to reproduce indefinitely.
The Pando aspen clone in Utah is thought to be 80,000 years old; some think it could be ten times older. It's hard to guess age, as counting tree rings isn't much help, and a long-term study would have had to start when humans were still mustering to leave Africa.
In an earlier study, Ally calculated the rate at which tiny genetic mutations -- slight, consistent copy errors in the signal of life, popping up when cells divide -- accumulated in 20 aspen clones in British Columbia. With that rate, she could sort the clones by age. In the latest study, published August 17 in PLoS Biology, Ally's team correlated age with pollen production.
As the clones got older, pollen counts and viability dropped. The oldest clone, estimated to be 10,000 years old, produced one-fourth the pollen of younger clones. Extrapolating the trend to its mathematical conclusion, it shouldn't take more than 20,000 years for pollen production to cease altogether.
At that point, the clone will still be able to copy itself through its roots, but it won't be able to make seeds. There won't even be a chance for pollen grains caught by a stray breeze to fertilize another, another, unrelated aspen, allowing the clone's lineage to persist in mixed form. The capacity for sexual reproduction will be gone.
Without seeds that can be picked up and deposited at a distance, the clone will be immortal but stuck in place. It will be vulnerable to disease, drought, fire, climate change or some other inevitability.
"You're at risk of going extinct if you lose your sexual fitness, because you have no other way of dispersing into a different environment," said Ally. "Once you get to the point of no return, where your sexual function is no longer capable of operating, then how do you persist in a changing environment?"
The findings are not meant to provide hard-and-fast numbers, but rather a general rule that likely applies to other clonal flora, such as the venerable King Lomatia tree of Tasmania and Michigan's ancient honey mushroom.
In a way, the seeds of the aspens' doom lie in their immortality's mechanism. While traits like root growth or leaf respiration or disease resistance are immediately subject to the pressures of natural selection, sex doesn't much matter for an individual tree. Even if seeds are defective, root clones suffice for reproduction. Sex-related mutations can freely accumulate -- and by the time seeds are needed, it's too late.
Sexual selection may still take place, only at some evolutionary scale beyond immediate comprehension, where generations are measured in millennia. For now, such speculation is beyond the scientific horizon.
As for the aspen, "I never really believed that anything could live forever," said Ally. "We just didn't have a way of showing how they aged."
Image: Scottks1/Flickr.
See Also:
- The Secret to Roundworm Longevity: Sex Cells
- The Oldest Trees on the Planet
- Biodiversity Explained by Ignoring the Forest for the Trees
Citation: "Aging in a Long-Lived Clonal Tree." By Dilara Ally, Kermit Ritland, Sarah P. Otto. Public Library of Science Biology, Vol. 8 No. 8, August 17, 2010.
Brandon Keim's Twitter stream and reportorial outtakes; Wired Science on Twitter. Brandon is currently working on a book about ecological tipping points.
